Polymeric membrane based separation processes such as reverse osmosis, pervaporation and perstraction are known in the art. In the pervaporation process, a desired feed component, e.g., an aromatic component, of a liquid and/or vapor feed is preferentially absorbed by the membrane. The membrane is typically exposed at one side to a stream comprised of a mixture of liquid feeds, and a vacuum is typically applied to the membrane at the opposite side so that the adsorbed component migrates through the membrane and is removed as a vapor from the opposite side of the membrane via a solution-diffusion mechanism. A concentration gradient driving force is established to selectively pass the desired components through the membrane from its feed or upstream side to its permeate or downstream side.
The perstraction process may also be used to separate a liquid stream into separate products. In this process, the driving mechanism for the separation of the stream into separate products is provided by a concentration gradient exerted across the membrane. Certain components of the fluid will preferentially migrate across the membrane because of the physical and compositional properties of both the membrane and the process fluid, and will be collected on the other side of the membrane as a permeate. Other components of the process fluid will not preferentially migrate across the membrane and will be swept away from the membrane area as a retentate stream. Due to the pressure mechanism of the perstraction separation, it is not necessary that the permeate be extracted in the vapor phase. Therefore, no vacuum is required on the downstream (permeate) side of the membrane and permeate emerges from the downstream side of the membrane in the liquid phase. Typically, permeate is carried away from the membrane via a swept liquid.
The economic basis for performing such separations is that the two products achieved through this separation process (i.e., retentate and permeate) have a refined value greater than the value of the unseparated feedstream. Membrane technology based separations can provide a cost effective processing alternative for performing the product separation of such feedstreams. Conventional separation processes such as distillation and solvent extraction can be costly to install and operate, in comparison with membrane process alternatives. These conventional based processes can require a significant amount of engineering, hardware and construction costs to install and also may require high operational and maintenance costs. Additionally, most of these processes require substantial heating of the process streams to relatively high temperatures in order to separate different components during the processing steps resulting in higher energy costs than are generally required by low-energy membrane separation processes.
A major obstacle to commercial viability of membrane separation technologies, particularly for hydrocarbon feeds, is to improve the flux and selectivity while maintaining or improving the physical integrity of current membrane systems. Additionally, the membrane compositions need to withstand the myriad of applications feed constituents, including alcohols.
Numerous polymeric membrane compositions have been developed over the years. Such compositions include polyurea/urethane membranes (U.S. Pat. No. 4,914,064); polyurethane imide membranes (U.S. Pat. No. 4,929,358); polyester imide copolymer membranes (U.S. Pat. No. 4,946,594); polyimide aliphatic polyester copolymer membranes (U.S. Pat. No. 4,990,275); and diepoxyoctane crosslinked/esterified polyimide/polyadipate copolymer (diepoxyoctane PEI) membranes (U.S. Pat. No. 5,550,199).
Another obstacle is the presence of alcohol in the feedstream, an increasingly frequent issue with government mandates and other incentives for adding alcohols to conventional hydrocarbon based fuels. Conventional polymer membranes suffer from instability in the presence of even small amounts of alcohol in the membrane feedstream. The present invention solves this problem.
Therefore there is a need in the industry for new membrane compositions with improved stability in processing alcohol containing feeds. There is also a need in the industry for new membrane compositions having high flux and selectivity for separating aromatics.